Propeller



June 24, 1930. G.- c. ENGSTRAND I ,767,786

PROPELLER Filed May 6, 192? 2 Sheets-Sheet 1 June 24, 1930. e. c. ENGSTRAND 1,767,785

PROPELLER Filed May 6, 1927 2 sheets-sheet 2 Patented June 24, 1930 UNHTED STATS FATE OFFEQE GUNNAR C. ENGS TRAND', OF BROOKLYN, NEW YORK, ASSIGNOB TO WILLIAM BRAAT, OF ENGLEWOOD, JERSEY rnorELLEn Application filed May 6, 1927. Serial No. 189,321.

This application is a Continuation in part larly relates to a screw propeller used for marine propulsion, although it will be clear that the invention is applicable to the design of propellers for aircraft or for fluid circulation in general.

It has heretofore been the aim of designers of screw propellers to minimize slip which has been considered as a necessary loss of power in the propulsion of ships. Propellers designed with this end in view have been particularly ineflicient when used at high speeds in connection with high speed otors, by reason of the setting up of cavitation by radial flow over the propeller blades and the rotationof the water leaving the propeller.

I have discovered, however, that the slip is not a measure of inefliciency of a propeller; in fact, a propeller adapted for a maximum efficiency at a given speed of operation must he designed to have a definite slip. Although slip results in less advance of the propeller relative to the water in one revolution, if the propeller is of proper design this is not accompanied by any waste of power. 7

The object of the present invention may, accordingly, be broadly stated to be the provision of a propeller having a maximum efficiency for high speed propulsion, and, of course, also for low speed propulsion.

In the attainment of the primary object, various ancillary objects follow, for example: the provision of a propeller which dis charges fluid straight backwardly without rotation or radial flow and the setting up of cavitation; the provision of a propeller having blades of proper width so as to accelerate all of the fluid in the propeller race while not reacting upon already accelerated fluid with loss of efficiency; and the provision of a propeller having the'suction and pressure sides properly corresponding with each other and properly designed for the load, speed of operation, etc. I

Other objects will become apparent from the following description read in conjunction with the accompanying drawings, in which:

Fig. 1 is a side view of a highvelocity marine propeller with the outlines of the a screw surfaces of which the blade surfaces are parts.

Fig. 2 is a rear elevation of the propeller of Fig. 1.

Fig. 3 shows cylindrical blade sections of the propeller of Fig. 1 developed in the conventional manner.

Fig. 4 is a view similar to Fig. 1 of a propeller designed to operate at a lower speed of rotation than the propeller of Fig. 1 but having the same speed of advance relative to water in the wake.

Figs. 5 and 6 are views respectively similar to Figs. 2 and 3 relating to the modification shown in Fig. 4c.

The construction of the improved propeller will be best understood from the following discussion of the considerations involved in the design of the propeller taken in their proper sequence.

An engineer undertaking the design of a propeller may have given the diameter of the propeller to be used, the horsepower available for propulsion, and the speed of the propeller shaft. By taking into consideration the design of the ship, he can calculate from experimentally determined data the maximum speed relative to the wake of the ship which will be obtainable using the type of propeller disclosed in this case. This speed may be designated 21rVn, in which 271-V is the advance per revolution of the propeller relative to the wake and n is the number of revolutions per minute of the propeller shaft.

It will be seen from this that propellers maybe designed for any given speeds of rotation of the propeller shaft (within practical limits) to produce the same speed of the ship relative to the wake. The propellers disclosed are designed to produce the same speed of advance of the same or equivalent ship, the propeller of Figs. 1, 2 and 3 being designed for a speed of rotation approximately 1.25 times the speed of rotation of the propeller of Figs. l, 5 and 6. It will be of a propeller of a different number oit blades or having blades the center lines of Which deviate from radii.

From considerations of the action of an' inclined plane in accelerating fluid through which it moves, since a propeller blade acts, ineffect, as an inclined plane, I have found that thepitch ofany helical line formed by taking-acylindrical section of the suction side of a propeller should be equal to the ,advanceper revolution ofthe propeller relative to the Wake (QwV) divided by the cosine of theangle of inclination of-the helical line to a plane transverse to theaxis in order to produce a straight backwards discharge of .theewater leavingthepropeller Without rotation or the setting upof cavitation. Designating the'pitch of anelement otthe suction side pf the propeller as Q rP, e thus have the relationship:

- tively indicated at 8, 9 and 10 While 11 indicates the path ofthe section of the blade ,Where, as Willbe pointed out later,the pitches of the pressure and suction sides of the blades-are made equal. Reference to Fig. 3 Will make clear the methodof laying out the suction surface of a blade. in this figure the cylindricalsections of the blade at various radii are de veloped into planes inthe conventional man er. The cylindricalelements of both suction and pressure surfaces are helices and accordingly the developments of these ,elelnentsibecome straightlines. It will be obvicue, therefore,,that theelements of both surfaces are portions of the helicesgenerated as paths relative to Water acted upon by the .resnective surfaces.

7 To determine the slope :of an element of the suction side .otthe blade at radius R, an

clearance is necessary "LO insure properscce 'is drawn to the are m intersectin the axis otrotation at B. Thevpoint B ofthis intersection Will be at distance P from the intersection 0 of the blade axis and axis of rotation. The angle the tangent makes .With

'the'blade axis is A, the angle of inclination of the element under consideration. From the mode of construction given, it will be obviousthat the relationship V=P cos A i satisfied.

It may be noted thatWhile-the circumferenceofa circle With radius Ris 2175B, the pitch is 275?, and the advance per revolution is 215. for simplicitythe factor 211' .may .be; dr pped throughout to simplifythe layout in Fig. 8.

The Water approaching the propeller has a velocityof QWVn when approachingthe suction side-and is accelerated-bythe suctionside to a velocityof discharge of 21rPn. The velocity of discharge from'the-suetion side-of one bladebecornes thevelocity of approach to thepressure side of thenext,succeedingblade. Accordingly, from theoretical consid rations as indicated above, 2771 the pitch ofan element or" the pressureside of a bla de, should equal QwP 1 divided bythe cosine of she-angle of inclinationof that elementotthe pressure side. That is, 2;;P

cos A V crVP=P cos A, fi /being the inclination. angle of the element of the pressure sidc under consideration.

To determine the .slopeof an element of the pressure side at radius R, an are n may be drawn concentric With-arc m and having a. radius equal. to P. A. tangent to arc n from the point S at distance-R from the axisof rotation measured along the blade axis will intersect-the axis of rotation at D at distu P from O. The angle this tangent malt s with the blade axis'is A" audit is obvious that P=P cos A.

The thickness ofthe blade' at any section 13 is, of course,determined from .consid' ration of the required strength at that secti n. The leadingand trailing edges o'it'the biade may be formed in any desired manner.

From theoretical considerations, I have found that the Width of anactive blade secticn should be equal to the clearanceiG b tween successive b ade paths measured g r- .pendicularly therebetween. Each of the surfaces should accordingly have a Width equal to 27V if tnree bladesare usedor,s1n general,

for a propeller having any number of blades,

the total w dthsof the, Y ctive surfaces should be QWV. This relat one iipof biadewidths eration of the water in the propeller race without a reaction of the blades upon water already accelerated.

Referring back to the mode of determining the slopes of the suction and pressure sides at radius R, it will be obvious that From this it will be seen that when R=V, the pitch of the suction side at radius B would become infinite. Similarly, when R=P, (that is when R=V the pitch of the pressure side at radius B would become iniinite. If the hub has a radius equal to, or greater than, V, this condition would not be reached on the suction side; or if the hub has a radius equal to, or greater than, V- 2, this condition would not be reached on the pressure side. In the propeller shown in Fig. 8, the hub has a radius greater than V so that the sections of the suction side from the outermost section 12 to the hub vary in accordance with the theoretical considerations. On the other hand the hub has a radius less than V- J2 and accordingly it is necessary to stop the variation of the inclinations of the ele ments of the pressure side before the critical radius is reached. Therefore the inner elements of the pressure side are given pitches equal to those of the corresponding elements of the suction side and are inactive since the velocity of approach of the water to such pressure elements is already that to which the elements are capable of theoretically producing acceleration. It will be seen from this construction that the sections between 12 and 18 taper so that the leading edge is of less wit th than the trailing edge; or, in other words, these sections are wedge-shaped. The sections between 13 and the hub are of equal thicl'aess from edge to edge.

The propeller 1 shown in Fi s. 4c, 5 and 6, is, as stated, designed for a lower speed of rotation than the propeller heretofore dis.- cussed, and consists of a hub 15 keyed to a shaft 16 and provided with a fair water nut 1. The blades 18 have suction sides 19 and pressure sides 20. Lines 21, 22 and 23 indicate the paths of the tips of the respective blades, while 24 indicates the path of the outermost section at which the suction and pressure sides have the same pitch.

In this latter modification, V is larger than in the former modification. The pitches of tie outermost elements of both suction and pressure sides are determined in the same manner as disclosed in connection with the modification of Figs. 1, 2 and 3. In this modification. however, the radius of the hub is less than V. Accordingly,'prior to the section where the radius decreases to V"\/ 2, that is, at section 26, the pitches of both the suction and pressure sides are made constant and equal to 279V and are accordingly rendered inactive. This arrangement is desirable inasmuch as an abnormally large hub offers increased resistance to passage through the water. Rendering the suction or pressure side or both inactive near the hub results in relatively small loss in efficiency since the greater portion of the work done by a propeller of this type is done by its outermost portions. 7

By adopting theconstruction described, it is found that there is no substantial tendency for flow to take place in a radial direction across the blades. This feature coupled with the straight backward discharge of water from the propeller eliminates the energy losses due to cavitation.

By following the modes of design of all of the elements of a propeller as outlined above, a propeller is.produced having both theoretically and in fact a maximum efficiency, the advantages being evidenced by actual use. It will be obvious, however, that adoption of various single features of the invention will result in considerable improvements in operation of propellers of conventional type.

The conventional propeller in general use at present embodies a screw surface of constant pitch constituting the pressure side, and a curved suction side, the blades being thickest at the center of the blades or adjacentthe leading edges of the blades so that there are produced blades having thick forebodies. Examination ofa blade of this type will disclose that the effective pitch of the suction side is greater than that of the pressure side, so that the water passing through the propeller is first accelerated by the suction side and then decelerated to the velocity of discharge determined byv the smaller pressure pitch. In using a propeller of this type at high speeds, the immoderately large initial acceleration will produce excessive suction slip, disturbance of the water, and cavitation with corresponding loss of energy which will not be retrieved when the water is decelerated to the discharge velocity. Furthermore, the blade width is improper. r

. The first component feature of advantage in the present invention is the provision of a propeller having transverse sections the suction sides of which have smaller pitches than the corresponding pressure sides so that the acceleration of the water from the entrance velocity to the discharge velocity takes place in steps. A high speed of operation may thus be attained, without immoderate acceleration of such degree as to produce cavitation being effected in either step. To attain these advantages, it will be obvious that the pitches of neither surface need vary with the radius. For example, both the suction and pressure approximately equal to the clearance between the paths of the corresponding elements of the suction side. Theoretically, of course, the width of the elements of each side should be equal to their corresponding clearances. Since this relationship of bladejwidth to clear} ance is founded on the basisof having each blade do its full share and no more in accelerating the water in the propeller race, it will be seen that this feature is also applicable to propeller blades although the pitches of the sides may be the same or different and either'constant or variable: By reason of the fact that the pitch of an element of the suction side in the disclosed type of propeller is a function of the radial location of the element it hao ens that the clearance throughout the'blade length'is constant and therefore that the blades are of constant width. If the pitches ofthe elements vary according 'to some other plan or are constant, it is ob-' vious that the clearancewill vary from tip to hub and accordingly the blade width should correspondingly vary. 7 I

While in the propellers disclosed the active portions of the pressure sides of the blades 3 diifer from the corresponding portions of the suction sides, it may under some'conditions be desirable to render the pressure sides totally inactive by making the pitches of elements thereof the same as the pitches of the corresponding elements of the suction sides, or to make them partially inactive by giving the .elements thereof pitches less than those disclosed as most desirable. In anyj'case, however, the water will be discharged straight backwards and without rotation. As a further obvious embodiment. the suction side might be renderedinactive by'giving it a constant pitchequal'to 21rV, in which case the pressure sideshould be formed as disclosed for the suction side.

As a corollary to the'theory discussed above follows a method whereby a propeller having a thick forebody maybe made to workerficiently; As has been stated, the water accelerated by the suction sideof a'propeller of this typeis decelerated'by the-pressure side to thedischarge-"velocity. If the pressure pitch'of a given section is madefequal to the product of the suctionpitch of'the section "and the cosine of the inclination angle of the pres sure side of the section underconsideration,

. the water in being decelerated to the discharge velocity develops a helpful torque at the propeller shaft without loss of energy while the water will be discharged straight backwards without rotation.

.Vhile theoretically the elements of the sides formed bytaking cylindrical sections of the blades are helices, it may bedesirable to approximate the theoretical contours of the surfaces by forming the surfaces of straight line elements to facilitate machining; likewise, the leading and trailing edges of the'blad'es may be suitably tapered or rounded to minimize resistance to passing throughthe water. As other errors may be introduced because of imperfectworkmanship or a desire to approximate in some manner, the theoretical construction tofacilitate manufacture, it will be understood that structures are included within the scopes of the appended claims which embody such deviationsas do not take the structure outside the spirit and scope of the invention. It will be clear from the above that, as stated in the beginning of this. specification, the various features of. the invention are applicable to'propellers for aircraft or for fluid circulation in generahand such propellers come within the scopes of the appended claims.

What is claimed is:

1. A propeller having the blades at all points as wide as the clearance between adj acent blade paths, substantially as described.

2. A propeller having the suction side shaped as a true screw surface and of a width at all points equal to the corresponding clearance between adjoining blade paths, substantially as described.

3. propeller having a plurality blades, each blade having an active surface the width of which at any radius is substantially equal to the clearance between adjacent blade paths at said radius,

4:. A propeller having a plurality of blades, each bladehaving an active suction surface the width of which at any radius is substantially equal to the clearance between adjacent blade paths at said radius.

5. A propeller having a plurality of blades, each blade having an active surface consisting of substantially helicalfelements,

the width of which surface at any radius is substantially equal to the clearancebetween adjacent blade paths at said radius.

6. A propeller having a pluralit Y of blades, each blade having' an active suction surfaceconsisting of substantially helical elements, the width of which surface at any radius is substantially equal to the clearance between adjacent blade paths at said radius.

7. A propeller having a' plurality of blades, each blade {having an active surface consisting of substantially helical elements,

the. width "of which surface at: any radiiis is substantially equal: to the-clearance [between the helices xofwhich the: elements of; adjacent 0 ,between the helices of Which the elements of adjacent blades at that radius are portions.

9. A propeller having a plurality of blades, each blade having an active surface the effective pitch of which varies With the radius so as to be at any radius substantially equal to a constant divided by the cosine of the inclination angle of the surface at that radius.

10. A propeller having a plurality of blades, each blade having an active suction surface the effective pitch of Which varies with the radius so as to be at any radius substantially equal to a constant divided by the cosine of the inclination angle of the surface :iat that radius.

11. A propeller having a plurality of blades, each blade having an active surface consisting of substantially helical elements, the pitches of such elements varying With the :iOaradius so as to be substantially equal to a constant divided by the cosine of the incli nation angle of the element under consideration.

12. A propeller having a plurality of "blades, each blade having an active suction surface consisting of substantially helical elements, the pitches of such elements vary ing with the radius so as to be substantially equal to a constant divided by the cosine of the inclination angle of the element under consideration.

13. A propeller having a plurality of blades, each blade having a surface the pitch of which varies With the radius so as to be at any radius substantially equal to the ef fective pitch of the opposite surface at the same radius divided by the cosine of the inclination angle of one of the surfaces at that radius.

14. A propeller having a plurality of blades. each blade having an active pressure surface the pitch of Which varies With the radius so as to be at any radius substantially equal to the effective pitch of the opposite suction surface at the same radius divided by the cosine of the inclination angle of the pressure surface at that radius.

15. A propeller having a plurality of blades, each blade having a surface consisting of substantially helical elements, the pitches of such elements varying With the radius so as to be at any radius substantially equal to the effective pitch of the opposite surface at the same radius divided by the cosine of the inclination angle of one. of the surfaces at; that:

radiiis.-

16. A propeller having a plurality ofv blades, eachblade having an active pressure surface consisting, of substantially helical elements, the pitches of suchelements varying with .the radius so'as. to beat any radius 1 I substantially equalto the'el'lective pitch of the -0pposite suction surface :-atthe .same

radius divided by the inclination angle of the element under consideration...

17. A propeller having a iplulflllty of blades, each ibladc having an active suction surface the eifective pitch of which varies with the radius so asto be. at. any radius substantiallyequalto aconstant divided by the.

cosine of theinclination angleof the suction.

surface at ithatradius, and each blade havin anactive pressure surface the pitch ofwhich varies Withthe radius so as to be at any radius substantially equal to the p tch.

of the-suction surface atthe. sameradius dis vided. by the cosine of the; inclinationv angle of the pressure surface at that, radius.

18. A propeller. having, a plurality, of

blades, each blade. havingan active suction: surface .consisting...of substantially helical elements, the pitches of such elements vary ingwiththe radius so as to be substantially; equal to a constant divided by. the cosineof.

the inclination angle of the element Lunder. consideration, and each blade having i110. tive pressure surface consistingof substan-.

tially helical elements, the pitches-of such.

elements varying With the radius so as to be at anyradius substantiallyequal to vthe pitch of the suction surface at the same radius di- 1 vided by the .inclination angleof the pres sure element under consideration...

19. A propeller having'a plurality of 'lades, each blade havingasuction'surface consisting of substantiallyhelical elements andhaving a pressure surface consistingofsubstantially helical elenients,-the helical element of the suction surface at a given radius having a smaller pitch than the helical element of the pressure surface at the same radius at theouter portions of the blade, but

having the same pitchattheinner portions of the blade.

20. A screw propeller having substantially wedgeshaped blade sections at the outer parts of the blades and blade sections with the pressure 1 side parallel.- With the suction side at the inner parts .of the blades.

21.-.A* propeller having a. pluralityiof blades, each blade having a suction surface consisting of substantially helical elements an (11 havinga pressure surface consisting of substantially helical elements.

22.-.A'- propeller having: a plurality. of

blades, each. bladeiha-vinga suctionsurface consisting vofisubstantially "helical elements and having a pressure surface consisting;- of;

substantially helical. elements, the Width .of

the blade at a given radius being substantially equal to the clearance between adjacent blade paths at said radius.

23. A propeller having a plurality of blades, each blade having a suction surface consisting of substantially helical elements and having a pressure surface consisting of substantially helical elements, the width of the suction surface at a given radius being substantially equal to the clearance between adjacent blade paths at said radius.

24. A propeller having a plurality of blades, each blade having an active surface the effective pitch of which varies with the radius so as to be at any radius substantially equal to a constant divided by the cosine of the inclination angle of the surface at that radius, the width of said surface at any radius being substantially equal to the clearance between adjacent blade paths at said radius.

25. A propeller having a plurality of blades, each blade having an active surface consisting of substantially helical elements, the pitches of such elements varying with the radius so as to be substantially equal to a constant divided by the cosine of the inclination angle of the element under consideration, the width of said surface at any radius being substantially equal to the clearance between the helices of which the elements of adjacent blades at that radius are portions.

26. "A propeller having a plurality of blades, each blade having an active suction surface consisting of substantially helical elements, the pitches of such elements varying with the radius so as to be substantially equal to a constant divided by the cosine of the inclination angle of the element under consideration, and each blade having an active pressure surface consisting of substantially helical elements, the pitches of such elements varying with the radius so as to be at any radius substantially equal to the pitch of the suction surface at the same radius divided by the inclination angle of the pressure element under consideration, the width of the said suction surface at any radius being substantially equal to the clearance between the helices of which the elements of adjacent blades at that radius are portions.

27. A screw propeller having wedge shaped blade sections around an inactive blade zone the radius of which is more than one-sixthof the advance of the propeller in one revolution.

28. A screw propeller having wedgeshaped blade sections at the outer parts of the blades and blade sections having the suction side substantially parallel with the pressure side at the inner parts of the blades, the pitch of the blades increasing towards the axis of rotation.

29. A screw propeller having wedgeshaped blade sections around azone the radius of which is more than one sixth the advance of the propeller per revolution, the pitch of the wedge-shaped sections increas ing towards the aXis of rotation.

GUN N AR 0. ENGSTRAND. 

